1. Field of the Invention
The invention relates to the field of semiconductor electronic and optoelectronic devices, and in particular to ways in which the temperature-dependent variations of the operating parameters of such devices may be eliminated, reduced or controlled.
2. Description of the Prior Art
The bandgap and refractive index of materials or semiconductors are temperature sensitive and determine the operating parameters of the device. For example, threshold current, differential efficiency, and wavelength of diode lasers all vary with temperature, and are usually stabilized by using bulky, expensive, and unreliable thermoelectric coolers. Filters, couplers, switches and wavelength routers are also temperature dependant. It is currently the common practice to mount semiconductor, optoelectronic or integrated optical components on a heat sink attached to a thermoelectric cooler and a temperature sensor. An external feedback circuit is used to control the temperature of the components in order to stabilize the operating parameters of the optoelectronic or integrated optic component in a temperature varying environment. See, for example, C. E. Zah et al., IEEE Journal of Quantum Electronics 30, 511-22 (1994). The technique of providing a controlled cooler is expensive, requires a relatively large space and has excessive power requirements.
Some operating parameters of certain devices may be less temperature sensitive by specific design. For instance, the threshold current and external differential efficiency of some diode lasers have been made less temperature sensitive by using particular active region and waveguide designs as discussed by Zah above. Stability in threshold current and external differential efficiency at a wavelength of 1.31 microns has been demonstrated, but wavelength temperature-drift remains. As closely spaced wavelength division multiplexed (WDM) communication systems develop, this wavelength sensitivity will become a limit to the system capacity.
In a quantum well laser, the wavelength of peak gain occurs at the band edge, so in a simple Fabry-Perot cavity, the lasing wavelength simply follows the bandgap as the temperature fluctuates. In GalnAsP/InP lasers, this results in a wavelength drift of 0.3-0.5 nm/C. The wavelength of diode lasers may be made less temperature sensitive by using Bragg filters for wavelength selection in semiconductor lasers, such as in DFB and DBR lasers as discussed in G. P. Agrawal et al., "Long Wavelength Semiconductor Lasers," Van Nostrand Company, New York (1986), and in silica or polymer waveguides, such as discussed by P. A. Morton et al., "High-Power, Narrow Linewidth, Stable Single-Mode Hybrid Laser, Optical Fiber Communication," Optical Society of America, Volume 4, pages 102-3, (1994); and G. D. Maxwell et al., "Semiconductor External-Cavity Laser with UV Written Grating in a Plain or Silica Waveguide, Optical Fiber Communication," Optical Society of America, pages 151-2, (1994). However, since the refractive index is dominated by the absorption at the band edge, even these structures will drift at 0.1 nm/C.
More recently, passive waveguides made from silica or polymers have been used which show very good temperature stability, such as described by Y. Kokubun et al., IEEE Phototonics Technology Letters 5, 1297-1300 (1993), but in this case, only the effective refractive index is stabilized and not the gain spectrum, and further, the method is not applicable to semiconductor devices.
Another current practice to temperature stabilize optoelectronic components is to couple them to passive dielectric waveguides which have an inherently reduced temperature sensitivity. The coupling may be entirely optical or the dielectric element may be included in a control loop. In the former case, only partial compensation may be achieved, since the semiconductor elements remain temperature sensitive. In the latter case, all the disadvantages of the temperature control loop are also present.
The effects of mechanical stress on the optical and electronic properties of semiconductors are well known. See for example A. R. Adams et.al., Semiconductor Science and Technology 5, 1194-1201 (1990). Stress produced by some monolithically integrated element has also been used to enhance the properties of some optoelectronic devices. For example, the stress from a deposited dielectric film has been used to create the waveguide region in a diode laser, as shown by P. A. Kirkby et al., Journal of Applied Physics 50, 4567-79 (1979). The stress from lattice-mismatched epitaxial layers has also been used to create quantum wire regions in semiconductor devices, as reported by I. H. Tan et al., Applied Physics Letters 59, 1875-77 (1991). However, never before has temperature-dependent stress been used to compensate for other temperature-dependent effects in semiconductor electronic or optoelectronic devices or components.
Semiconductor optoelectronic components, such as diode lasers are finding increased use in data and telecommunications as well as in optical sensors and optical computing. Because the energy band gap of semiconductors is dependent upon temperature, all properties related to band gap are also temperature dependent. In particular, the gain or loss spectrum and the index of refraction are temperature dependent. Because of this temperature dependence, the operating wavelength of diode lasers, with or without Bragg reflectors, and the operating wavelength of components, such as filters, detectors, switches and wavelength routers, is also necessarily temperature dependent. This dependence requires that the components be very carefully temperature controlled in many applications. It is expected that in fiberoptic communication systems employing many closely spaced wavelength channels, the requirement for optical temperature stability will be even greater. Therefore, it is the object of the invention to reduce or eliminate entirely, the temperature sensitivity of the optoelectronic component so that external temperature control of the optoelectronic or integrated optic component becomes unnecessary.
Therefore, what is needed is some type of apparatus or methodology whereby a semiconductor, optoelectronic or integrated optic component can be made less temperature sensitive in a manner which is adaptable for use in most, if not all, system applications, which provides stabilization of the bandgap, gain spectrum, and refractive index, and is also applicable to semiconductor electronic devices.